The invention pertains to a method and a device for determining the 3D shape of an object. The object may also be referred to as the measured object. The method and the device make it possible to determine, in particular, the spatial 3D coordinates of surface points of the object or measured object, respectively.
Tactile and optical methods are primarily utilized for determining the shape of objects. In comparison with tactile-scanning coordinate measuring machines, optical 3D measuring methods make it possible to carry out a contactless, full-surface and time-efficient measurement of complicated object shapes of various sizes.
In the meantime, a series of optical sensors with different properties were developed for the field of optical metrology. For example, the complete 3D shape of surfaces is frequently measured with optical triangulation methods (strip projection, Moire). However, various 3D measuring arrangements with coherent light (laser scanning method, interferometric contouring method) are also known.
Tactile and optical methods are required in the field of three-dimensional coordinate metrology, in full-surface 3D nominal/actual measurements and surface tests on technical objects, and in the inspection and position control of components, in particular, of motor vehicles or modeling and tool moulds.
Due to the limited measuring volumes and the limited resolution capacity of available three-dimensional coordinate measuring devices (in the following referred to as 3D-KMG), it is necessary to assemble the measuring data of individual partial measuring fields into the 3D shape of the entire object similar to a mosaic, namely with the least possible loss in accuracy and surface structure resolution, when measuring highly structured objects and large-surface objects.
There exist various (tactile and optical) measuring methods and 3D sensors for measuring 3D coordinates and for determining the 3D shape of objects. The variety of proposed measuring and evaluation strategies is exceptionally broad. With respect to optical measuring methods, important measuring principles are the triangulation method and the interferometry method. The triangulation and the contour measuring method are of the utmost practical importance for determining shapes and for surface controls. In many measuring tasks, a full-surface and consequently simultaneous acquisition of contour data of the entire surface is advantageous. Imaging contour measuring methods (strip projection, Moire, interferometric contouring method) are used for such tasks in the form of so-called field measuring methods. With respect to full-surface optical measuring systems, strip projection methods with a matrix camera and a projector for non-coded or coded strips are known, wherein the three-dimensional coordinates of the surface points are calculated from the image coordinates of the camera image and the strip numbers detected on the respective image coordinate based on geometric models of the measuring arrangement. In other known methods that are based on photogrammetry, several cameras and projectors are utilized. Laser triangulation methods that operate point-by-point and line-by-line are, in part, supplemented with expensive scanning and handling mechanisms for realizing a full-surface scanning. Imaging triangulation methods from the field of photogrammetry make it possible to measure a series of individual points with high accuracy.
All 3D measuring methods and systems have a common problem that is caused by the measuring principle, namely that an individual surface measurement by means of 3D-KMG is not sufficient for determining the complete shape of an object to be measured, e.g., in an allround measurement. This is the reason why objects are recorded and measured from several directions with a movable 3D-KMG or several 3D-KMG. The spatial arrangement of the 3D-KMG relative to the object may change, e.g., when repositioning the measuring device or the measured object. If the corresponding positions of the 3D-KMG relative to the measured object are known, the corresponding individual measurements can be linked similar to a mosaic in order to reconstruct the complete object. The 3D object coordinates acquired in the respective device coordinate system (in the following referred to as device-KOS) by means of individual measurements are transformed into the fixed coordinate system of the 3D measuring arrangement or the reference coordinate system (in the following referred to as reference-KOS) by means of a geometric transformation rule.
The following steps are carried out:    1. Determining the unknown parameters of the transformation between the device-KOS and the reference-KOS;    2. Carrying out the transformation into the data sets of the individual measuring fields, i.e., calculating the 3D coordinates in the reference-KOS with the aid of the calculated transformation parameters.
One metrological problem can be seen in the fact that inaccuracies in the position values result in gaps between the individual fields, as well as in a reduced accuracy, over the complete object surface. The invention aims to solve this problem.
Various methods and system solutions are known for determining the positions of the measuring device and for linking the individual measuring fields.
In addition to tactile probes, optical 3D sensors are used which are positioned in the required measuring positions and aligned relative to the object by means of a mechanical guidance system (e.g., multiaxis CNC, coordinate measuring machines, measuring arms, robots). The position values are predetermined for the guidance system (internal glass measuring rods), wherein the transformation rule to be applied for linking the individual measuring fields is determined from said position values. The accuracy of the entire measurement requires complicated designs of the motion mechanics. The high constructive and monetary expenditure for realizing the guidance and measuring system is considered disadvantageous. Despite the high expenditures, significant residual errors and gaps between the measuring data remain, in particular, in multiaxis positioning systems. The definition of the path movement, the measuring volume and the displacement speed for the CNC machine requires a significant expenditure of time. Another disadvantage can be seen in the fact that the measured object needs to be transported to the measuring system, i.e., a mobile measurement cannot be realized with systems of this type. Special embodiments contain sensors that are mounted on a movable “measuring arm”, (e.g., an articulated arm). Although the simple portability of these devices makes it possible to carry out mobile measurements, the object measuring range is limited to approximately 1 m.
There also exist systems in which the sensor (or the tactile probe) is guided over the object manually or with the assistance of motion mechanics while measurements are carried out. The movement and position of the sensor is detected by means of an external (e.g., optoelectronic) reference measuring system that is fixed in space and the position data of which are used for linking the individual measurements. This solution requires a high accuracy of the reference measuring system. The mobility of the system is limited due to the fact that the sensor always needs to be situated within the measuring range of the reference measuring system.
There also exist photogrammetric methods and systems that are able to determine the spatial position of a camera (or a projector) relative to several visible reference points, the coordinates of which in space are predetermined (spatial step backward). In other known photogrammetric methods and systems, it is possible to determine several spatial positions of a camera (or a projector) or the spatial positions of several cameras (or projectors) relative to one another as long as the halftone and phase images recorded from different directions contain several common measuring points, e.g., in the form of measuring markings, the coordinates of which do not have been known beforehand (bundle compensation). In this method, the coordinates of these measuring points are calculated in addition to the positions of the cameras or projectors (simultaneous calibration). A length reference is introduced as the scale.
There also exist photogrammetric orientation methods that make it possible to determine the spatial position of an optoelectronic 3D sensor (consisting of at least one camera and one projector), a camera or a projector relative to several visible reference points, the coordinates of which in space are predetermined. A reference network of individual measuring points which is arranged stationarily referred to the measured object makes it possible to assemble the measuring data of individual partial measurements. It is known that such reference points, e.g., in the form of measuring markings, may be arranged on the object itself or outside the object, e.g., on external measuring cages or connecting links (DE 198 40 334 A1, DE 195 36 294 A1).
Prerequisites for a sufficiently accurate determination of the position of optoelectronic sensors by means of a reference network are a minimum of three to four measuring markings in a sensor recording, as well as their largely uniform distribution in the measuring field. Consequently, a large number of coded and non-coded measuring markings needs to be used for measuring large objects. The positioning of these measuring markings and, if applicable, their spacing consequently are very time-consuming (high expenditure of labor). In addition, significant parts of the surface of the measured object are covered by the adhesive markings or the measuring cage or connecting link used. If the minimum number of measuring markings or their uniform distribution in the measuring field cannot be ensured when measuring objects with a complex structure, i.e., objects that are not two-dimensional, the object cannot be completely measured. When measuring objects with a size of more than 1 cubic meter, measuring cages or connecting links become difficult to handle and transport due to the required large size and heavy weight. This means that the advantage of a mobile measuring system is lost. The accuracy is reduced due to an insufficient stability of the connecting link, in particular, if its mass is reduced. The calibration of the markings always requires the utilization of an additional measuring system and consequently is associated with an increased expenditure of labor.
In other known methods, a so-called matching (also referred to as registering) of 3D data sets that are primarily obtained with the aid of optical field measuring methods is carried out. In this case, the positions of two 3D-KMG relative to one another or two positions of one movable 3D-KMG are determined from overlapping 3D data sets. These methods are also used for matching measured 3D data sets with predetermined CAD models of these objects in order to carry out a nominal/actual comparison. When matching 3D data sets, numerical compensation methods are applied which calculate the required transformation from one 3D data set into the other 3D data set, i.e., the transformation parameters. This makes it possible to carry out the transformation between the spatially changed device-KOS. One device-KOS is usually defined relative to the reference-KOS such that the 3D data of different device-KOS can be transformed to this reference-KOS. This results in a uniform surface description with 3D object coordinates in the reference-KOS.
Matching methods require a sufficient overlap between the individual measuring fields and a sufficient structuring of the object surface. The individual transformations can otherwise not be determined in a sufficiently definitive and accurate fashion. This means that the limited accuracy over the entire object and the dependence of the measuring result quality on the structure of the object represent two of the more significant disadvantages of these methods. This method is entirely unsuitable for smooth object surfaces (no definitive solution of the transformation).